This invention relates to techniques and apparatus for wireless communication using Direct Sequence Spread Spectrum (DSSS) techniques.
In the transmitter of a direct-sequence spread spectrum communication systems, a carrier waveform is modulated by a data sequence x(n) and by a spreading sequence or code sequence C(n). The code sequence may be a pseudo-noise (PN) sequence, such as a maximum length sequence (m-sequence). The PN sequence is used to reduce the sensitivity of the communication channel to noise, reduce the power spectral density of the signal and to allow multiple communication channels to operate simultaneously. In the latter case, each channel is assigned its own PN code sequence, so the technique is called code-division multiple access (CDMA).
In the receiver the data signal is recovered by removing the carrier wave and then correlating the received signal with the PN code sequence used for transmission. The recovery of the data signal in the receiver is hindered by interference. The correlation process removes much of the uncorrelated interference, but other interference is due to the signal reaching the receiver by indirect propagation paths. This multipath interference is correlated with the PN code sequence. The presence of multipath interference makes it more difficult to align the received signal with the PN code sequence and increases the likelihood that a signal is incorrectly decoded. Equalizers can be used to mitigate the effects of multipath interference, but these require large amounts of computation and are not suitable for a low cost receiver.
In a DSSS location system, the distance between the transmitter and the receiver is determined from the propagation time, which must be determined accurately for an accurate location to be determined. However, multipath interference can significantly degrade the accuracy of the propagation timing estimation, and hence the location cannot be determined accurately in a multipath interference environment.
In view of the preceding remarks, it is clear that there is an unmet need in the art for a low cost DS-CDMA receiver that can accurately decode a signal and determine the propagation time in the presence of multipath interference.